Microporous polyolefin composite film with a thermally stable porous layer at high temperature

ABSTRACT

Provided is a microporous polyolefin composite film with a thermally stable porous layer at high temperature, particularly, to the microporous polyolefin composite film in which the thermally stable porous layer at high temperature, which contains organic or inorganic particles and heat-resistant polymer having aromatic ring in main chain and also having a melting temperature or a glass transition temperature of 170 to 500° C., is formed on one surface or both surfaces of a polyolefin microporous film by a phase separation, wherein the composite film with the porous layer has a permeability of 1.5×10 −5  to 20.0×10 −5  Darcy, a meltdown temperature of 160 to 300° C., a MD/TD shrinkage of 1 to 40% at a temperature of 150° C. for 60 minutes.

TECHNICAL FIELD

The present invention relates to a microporous polyolefin composite filmhaving excellent permeability and also excellent thermal stability inhigh temperature electrolytes, and more particularly, to a microporouspolyolefin composite film with a thermally stable porous layer at hightemperature, which is used as a separator for a high-capacity/high-powerlithium secondary battery.

BACKGROUND ART

A polyolefin-based microporous film has been widely used as a batteryseparator, a separator filter, and a membrane for microfiltration, dueto its chemical stability and excellent physical properties. Meanwhile,for the battery separator, the microporous structure is required to havea spatial separation function between positive and negative electrodesand a microporous structure for high ionic conductivity. Recently, ithas been further required to enhance the characteristic of the separatorfor the thermal stability and electrical stability upon charging anddischarging of the secondary battery, according to the tendency of thesecondary battery toward the high-capacity and high-power. In case ofthe lithium secondary battery, if the thermal stability of the separatoris deteriorated, the separator may be damaged or deformed by increase oftemperature in the battery and thus an electrical short may occurbetween the electrodes. Therefore, there is a risk that the battery maybe overheated or ignited. The thermal stability of the battery isaffected by shutdown temperature, meltdown temperature, high temperaturemelt shrinkage and the like.

The separator having the excellent thermal stability at high temperatureis prevented from being damaged at the high temperature, therebypreventing the electrical short between the electrodes. If theelectrical short occurs between the electrodes due to dendrite generatedduring charging and discharging processes of the battery, the batterygenerates heat. At this time, in case of the separator having theexcellent thermal stability at high temperature, it is prevented thatthe separator is damaged and thus the battery is ignited or exploded.

In order to increase the thermal stability of the separator, there havebeen proposed a method that crosslinks the separator, a method that addsinorganic particles, and a method that mixes a heat-resistant resin witha polyolefin resin or forms a coating layer.

The crosslinking method of the separator is disclosed in U.S. Pat. No.6,127,438 and U.S. Pat. No. 6,562,519. In these methods, a film istreated by electron beam crosslinking or chemical crosslinking. However,in case of the electron beam crosslinking, there are some disadvantagessuch as necessity for an electron beam crosslinking apparatus, alimitation of production speed, a variation in quality according tonon-uniform crosslinking. And in case of the chemical crosslinking,there are also some disadvantages in that it has complicated extrudingand mixing processes, and a gel may be generated at a film due to thenon-uniform crosslinking.

Meanwhile, in U.S. Pat. No. 6,949,315, there is disclosed a method ofenhancing the thermal stability of the separator by mixing an ultra highmolecular weight polyethylene with inorganic particles like titaniumoxide of 5 to 15 weight %. However, in this method, there are somedisadvantages such as increase of extruding load, deterioration of theextruding and melt-kneading ability, and occurrence of incompletestretching due to using of the ultra high molecular weight polyethylene,as well as inferiority in the mixing, variation in quality andgeneration of pinholes due to using of inorganicparticles. Further,physical properties of the film are also deteriorated due to lack ofinterface compatibility between the inorganic particles and the highmolecular resin.

In U.S. Pat. No. 5,641,565, there is disclosed a method in which anexcellent heat-resistant resin is melt-kneaded. In this method, an ultrahigh molecular weight resin having an average molecular weight of1,000,000 or more is needed to prevent deterioration of the physicalproperties due to adding of polyethylene, polypropylene andinorganicparticles. Further, since an additional is also needed toseparate and remove the used inorganicparticles, the manufacturingprocess is very complicated.

In Japanese Patent Publication No. 2004-161899, there is disclosed amicroporous film which contains polyethylene and non-polyethylenethermoplastic resin having excellent heat-resistance and being notcompletely melted but minutely dispersed when being mixing with thepolyethylene. However, there is a disadvantage that the microporous filmmanufactured by this method has a non-uniform thickness due toparticulate heat-resistant resin. If the microporous film has thenon-uniform thickness, the defective proportion in assembling of batteryis increased and thus the productivity is reduced. Also, after theassembling of battery, an electrical short occurs, thereby deterioratingsafety.

In U.S. Pat. No. 5,691,077 and Japanese Patent Publication No.2002-321323, there are disclosed methods of forming additionally heatresistant layer on a polyolefin-based microporous film. In thesemethods, a polypropylene layer is provided by a dry or wet process, buta heat-resistant layer is stretched and it is difficult to basicallyprevent heat shrinkage due to limitation of a melting point ofpolypropylene. Therefore, there is limitation in manufacturing of a highheat-resistant separator. Further, in Korean Patent Publication No.2007-0080245 and International Publication No. WO2005/049318,polyvinyldene fluoride copolymer that is a heat-resistant resin is usedas a coating layer so as to enhance the heat-resistance of the separatorand the thermal stability of the battery. However, since the resin iseasily dissolved or gelled in an organic solvent such as propylenecarbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate,and ethyl methyl carbonate, which is used as a non-aqueous electrolyteof a battery, there is limitation in enhancing the thermal stability ofthe battery.

In Japanese Patent Publication No. 2002-355938, there is disclosed amicroporous polyolefin composite film in which a high heat-resistantresin is used. In this film, the high heat-resistant resin is applied toa polyolefin microporous film by the phase separation. However, it isdifficult to provide efficient permeability by a pore forming method inwhich a single resin is phase-separated by a dry process when forming acoating layer of the film. Further, since phase separation size anduniformity are considerably changed according to the drying conditionssuch as humidity, temperature and so on, there is limitation inmanufacturing the separator having uniform quality.

With respect to the heat-resistance as one of the main characteristicsof the battery separator, the conventional methods have a limitation inthe heat-resistance of the resin itself, or the applying of theheat-resistant resin does not contribute to the improvement of theheat-resistance of the separator. And other physical properties like gaspermeability are low or do not mentioned, and also the qualityuniformity is poor. Further, when the separator manufacture by theconventional methods are actually applied to the battery, there is aproblem that it is not provide constant thermal stability under theconditions such as high temperature, high voltage and organicelectrolytes.

DISCLOSURE Technical Problem

An object of the present invention is to provide a microporouspolyolefin composite film with a thermally stable porous layer at hightemperature, which has excellent permeability and also excellent thermalstability in high temperature electrolytes, and more particularly, toprovide a separator which is proper to high power/high capacity of abattery.

Technical Solution

To achieve the above objects, the present invention provides amicroporous polyolefin composite film in which a thermally stable porouslayer at high temperature, which contains organic or inorganic particlesand heat-resistant polymer having aromatic ring in main chain and alsohaving a melting temperature or a glass transition temperature of 170 to500° C., is formed on one surface or both surfaces of a polyolefinmicroporous film by a phase separation, wherein the composite film withthe porous layer has a permeability of 1.5×10⁻⁵ to 20.0×10⁻⁵ Darcy, ameltdown temperature of 160 to 300° C., a machine direction (MD) and atransverse direction (TD) shrinkage of 1 to 40% at a temperature of 150°C. for 60 minutes.

Further, the present invention provides a method of manufacturing thethermally stable porous layer at high temperature, including (1)preparing a mixed solution by mixing organic or inorganic particles anda nonsolvent of heat-resistant polymer with a solution in which theheat-resistant polymer is dissolved in a solvent; (2) coating the mixedsolution on one surface or both surfaces of a polyolefin microporousfilm; (3) forming the thermally stable porous layer by drying andphase-separating the coated polyolefin microporous film; and (4)removing the solvent and nonsolvent remained in the thermally stableporous layer by drying or extracting.

Further, the present invention provides a separator for a lithiumsecondary battery, which contains the microporous polyolefin compositefilm with the thermally stable porous layer at high temperature.

Further, the present invention provides a lithium secondary batteryhaving the separator.

Hereinafter, the present invention will be described more fully.

In the microporous polyolefin composite film of the present invention, athermally stable porous layer at high temperature, which containsheat-resistant polymer having aromatic ring in main chain and alsohaving a melting temperature or a glass transition temperature of 170 to500° C. and organic or inorganic particles having a size of 0.01 to 2μm, is formed on one surface or both surfaces of a polyolefinmicroporous film by a phase separation, wherein the entire compositefilm including the layer has a permeability of 1.5×10⁻⁵ to 20.0×10⁻⁵Darcy, a meltdown temperature of 160 to 300° C., a MD/TD shrinkage of 1to 40% at a temperature of 150° C. for 60 minutes. Preferably, in orderto provide such physical properties, an entire thickness of the coatinglayer is 0.1 to 1.0 times that of the polyolefin microporous film, and abonding force between the coating layer and the polyolefin microporousfilm is 0.1 to 1.0 kgf/cm.

It is preferable that the polymer in the coating layer has a meltingtemperature or a glass transition temperature of 170 to 500° C. If themelting temperature is less than 170° C., it is not possible to securethe thermal stability enough to endure a rapid rise in temperature byinternal short of the battery, and if the melting temperature is morethan 500° C., too much energy is consumed when melting the polymer andthe thermal stability at high temperature is no longer enhanced.Preferably, the polymer contains aromatic ring in main chain. In thiscase, since the melting temperature or the glass transition temperatureis increased due to increase in rigidity of the polymer chain, the heatresistance is increased. Further, due to the hydrophobicity of thearomatic ring, the polymer is not easily dissolved or gelled in theorganic electrolytes such as propylene carbonate, ethylene carbonate,diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. andthe coating layer is stably maintained at high voltage and hightemperature when applied to the battery.

According to the present invention, the organic or inorganic particleshaving a size of 0.01 to 2 an is contained together with theheat-resistant polymer. Various organic or inorganic particles can beselected according to various purposes of increase in a impregnationrate of a liquid electrolyte with respect to a separator, increase inphysical strength of the coating layer, increase in a porosity of thecoating layer, increase in heat resistance of the separator, andprevention of an electrical short by securing a space between electrodesupon an abnormal condition of the battery. The organic or inorganicparticles is used for these purposes independently or combined. Theparticle size is 0.01 to 2 μm, more preferably, 0.05 to 1 μm. If theparticle size is less than 0.01 μm the particles may close up pores inthe surface of the polyolefin microporous film and thus the permeabilitymay be deteriorated, or otherwise the particles may be buried in thepolymer after the phase separation and thus characteristic of theparticles may be not expressed. On the other hand, if the particle sizeis more than 2 μm, a final separator may have a non-uniform thickness,and it is difficult to secure the bonding force with the polyolefinmicroporous film, and also since the efficiency is lowered due toreduction of a surface area, there is difficulty in dispersion.

The contained organic particles include one or more of polyvinyldenefluoride (PVdF), polytetrafluoroethylene (PTFE), polyurethane,polymethylpentene (PMP), polyethylene terephthalate (PET), polycarbonate(PC), polyester, polyvinyl alcohol (PVA), polyacrylonitrile (PAN),polymethylene oxide (PMO), polymethyl methacrylate (PMMA), polyethyleneoxide (PEO), cellulose and so on, and the inorganic particles includeone or more of an oxide, a hydroxide, a sulfide, a nitride, a carbideand the like of at least one of metallic or semiconductor elements suchas Si, Al, Ca, Ti, B, Sn, Mg, Li, Co, Ni, Sr, Ce, Zr, Y, Pb, Zn, Ba.Further, alone or a mixture of the organic and inorganic particles maybe used.

Meanwhile, in case that the polymer of the coating layer does not form aporous structure or closes up the pores of the polyolefin microporousfilm during the coating process, the gas and ion permeability isdeteriorated, and thus electric characteristics of the battery such ashigh-rate characteristic, charging and discharging characteristic,low-temperature characteristic and cyclability are lowered. At thistime, the gas permeability is 1.5×10⁻⁵ to 20.0×10⁻⁵ Darcy, preferably,2.0×10⁻⁵ to 10.0×10⁻⁵ Darcy. If the gas permeability is less than1.5×10⁻⁵ Darcy, the ion permeability is lowered, and thus the electriccharacteristics are deteriorated, and if the gas permeability is morethan 20.0×10⁻⁵ Darcy, the gas permeability is too high, and thus safetyof the battery is lowered. It is preferable that the entire thickness ofthe polymer coating layer is 0.1 to 1.0 times, more preferably, 0.2 to0.6 times that of the polyolefin microporous film. If the entirethickness of the polymer coating layer is less than 0.1 times, it is notpossible to prevent the heat shrinkage and fracture at high temperature,and if entire thickness of the polymer coating layer is more than 1.0times, strength of the entire microporous film may be lowered due tolower strength of the coating layer than the stretched polyolefinmicroporous film, and this may result in deterioration of stability ofthe battery. Further, if a pore size of the coating layer having a thickthickness is not properly controlled, it may exert a bad influence onpower and long-term performance of the battery.

In order to enhance the thermal stability of the lithium secondarybattery, it is preferable that the separator has high meltdowntemperature. In the present invention, the separator has a meltdowntemperature of 160 to 300° C. The meltdown temperature is affected bythermal property of a separator material and stability in the organicelectrolyte. The polyethylene microporous film has a meltdowntemperature of 150° C. or less due to a limitation of a melting pointthereof. Even in case of a separator containing a high heat-resistantfluorine resin having a melting temperature of a glass transitiontemperature of 170 to 500° C. or a highly crystalline resin having astrong hydrogen bond, it is difficult that the meltdown temperature isincreased to 160° C. or more when dissolved or gelled in the organicelectrolytes. Since the coating layer contains aromatic ring in mainchain, the microporous polyolefin composite film of the presentinvention has excellent thermal property, and since the microporouspolyolefin composite film is stable with respect to the organicelectrolytes due to the hydrophobicity of the aromatic ring havinghydrocarbon group, it has a high meltdown temperature. Therefore, if apolymer containing aromatic chain having a melting temperature of aglass transition temperature of 170° C. or more is coated on thepolyolefin microporous film, the meltdown temperature is increased to160° C. or more. Therefore, if the meltdown temperature of the compositefilm is 160° C. or less, the thermal property of the composite film isdeteriorated, and if the meltdown temperature is 300° C. or more, theefficiency is no longer improved corresponding to the rise intemperature.

The MD/TD shrinkage is 1 to 40%, preferably, 2 to 30% at a temperatureof 150° C. for 60 minutes. Like in the meltdown temperature, the MD/TDshrinkage at a temperature of 150° C. shows the high temperaturestability of the separator. If the shrinkage is less than 1%, asinternal temperature of the battery is increased, the heat shrinkageoccurs. Thus, the two electrodes are exposed and an electrical shortoccurs between the electrodes, and fire and explosion occur. If theshrinkage rate is more than 40%, the physical properties aredeteriorated due to the excessive heat shrinkage. The MD/TD shrinkage isaffected by the thermal property of the separator material and anorientation degree of the resin. The polymer layer of the presentinvention is characterized by an excellent shrinkage at high temperaturesince a coating material has high thermal property and the polymer ofthe coating layer has a low orientation degree. In order to obtain theabove shrinkage, it is preferable that a bonding force between thecoating layer and the polyolefin microporous film is 0.1 to 1.0 kgf/cm.Although the coating layer has the excellent heat resistance andshrinkage at high temperature, if the bonding force is less than 0.1kgf/cm, it is not possible to prevent the shrinkage of the polyolefinmicroporous film, and a risk of the electrical short in the battery isincreased. If the bonding force is more than 1.0 kgf/cm, the increasedbonding force is not led to an effect of reducing the shrinkage.

Further, to achieve the above objects, a method of manufacturing themicroporous polyolefin composite film according to the present inventionmay include the following processes.

The method of manufacturing the microporous polyolefin composite filmaccording to the present invention includes:

(A) preparing a polyolefin microporous film using a compositioncontaining a polyolefin resin;

(B) coating a solution, in which a high heat-resistant resin isdissolved, on one surface or both surfaces of the polyolefin microporousfilm;

(C) phase-separating a composition contained in a coating layer so as toform the coating layer having pores after the coating; and

(D) drying the porous coating layer so as to remove components exceptthe composition of the coating layer.

Speaking more detailedly, it is preferable that the polyolefinmicroporous film is a single layer type or two or more laminated layerstype formed of polyethylene, polypropylene, polybutylene, and acopolymer thereof or a copolymer in which alpha-olefin comonomer havinga carbon number of 5 to 8 is contained in the polyolefin, and a mixturethereof. The materials are not specially limited, but it is preferableto contain high density polyethylene and polypropylene for the purposesof facility of manufacturing the separator, high strength, propershutdown temperature or meltdown temperature.

The proper shutdown temperature is 120 to 140° C. If the shutdowntemperature is less than 120° C., the pores of the separator may beclosed even by a relatively small temperature rise, and the battery isfailed. If the shutdown temperature is more than 140° C., it is notpossible to prevent the ignition and explosion generated by boiling ordecomposing of the organic electrolytes. It is preferable that themeltdown temperature is 140 to 200° C. If the meltdown temperature isless than 140° C., a temperature section, in which the pores are closed,is short when battery temperature is increased, and thus it is notpossible to efficiently prevent the abnormal behavior of the battery,and if the meltdown temperature is more than 200° C., the efficiency isnot improved corresponding to the increased temperature.

If necessary, some additives for particular purposes, such asantioxidant, UV stabilizer, and antistatic agent, may be added to thecomposition within a range that the characteristics of the separator arenot deteriorated remarkably.

The polyolefin microporous film may contain organic or inorganicparticles for the purposes of formation of the pores, enhancement of theheat resistance and impregnation of the organic electrolyte and thelike.

The organic or inorganic particles have a size of 0.01 to 2 μm.Preferably, the particle size is 0.01 to 2 μm, more preferably, 0.05 to1 μm. If the particle size is less than 0.01 μm, the particles may closeup the pores formed in the surface of the polyolefin microporous filmand the permeability is lowered, and also since the particles are buriedin the polymer after the phase separation and thus the characteristic ofthe particles may be not expressed. Meanwhile, if the particle size ismore than 2 μl, a final separator may have a non-uniform thickness, andit is difficult to secure the bonding force with the polyolefinmicroporous film, and also since the efficiency is lowered due toreduction of a surface area, there is difficulty in dispersion.

The organic particles include one or more of polyvinyldene fluoride(PVdF), polytetrafluoroethylene (PTFE), polyurethane, polymethylpentene(PMP), polyethylene terephthalate (PET), polycarbonate (PC), polyester,polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polymethylene oxide(PMO), polymethyl methacrylate (PMMA), polyethylene oxide (PEO),cellulose and so on, and the inorganic particles include one or more ofan oxide, a hydroxide, a sulfide, a nitride, a carbide and the like ofat least one of metallic or semiconductor elements such as Si, Al, Ca,Ti, B, Sn, Mg, Li, Co, Ni, Sr, Ce, Zr, Y, Pb, Zn, Ba. Further, alone ora mixture of the organic and inorganic particles as well as the organicparticles and the inorganic particles may be used.

There is not limitation in the method of manufacturing the polyolefinmicroporous film, but it is preferable to include one or more followingprocesses:

(a) a process of melting and mixing a polyolefin resin in diluent,which, can be mixed with polyolefin resin at high temperature, so as toform a sheet, and stretching the sheet after phase-separating so as toform a film, and extracting the organic solvent by using a volatilesolvent, and then drying and heat-setting the film.

(b) a process of melting a polyolefin resin so as to form a sheet, andstretching the sheet at low or high temperature so as to exfoliate aninterface between crystals and form film and pores, and thenheat-setting the film.

(c) a process of mixing organic or inorganic particles having a highermelting temperature than the polyolefin resin and stretching a sheet soas to exfoliate an interface between the resin and the particle and formfilm and pores, and extracting the particles or heat-setting the film ina state of containing the particles.

In order to enhance the heat resistance and strength of the polyolefinmicroporous film and also enhance the stability in the organicelectrolytes, a process of mixing monomer or oligomer having anunsaturated bonding group, and polymerizing and chemically cross-linkingthe mixture by using heat energy or ionizing radiation, or cross-linkingthe polyolefin alone or together with an initiator by using the ionizingradiation and the like may be included. The cross-linking can be carriedout at any time, e.g., after forming the sheet, before and after thestretching, before and after the extracting, and before and after theheat-setting, within a range that the basic physical properties of thepolyolefin are not deteriorated.

Before forming the coating layer on the polyolefin microporous film, aprocess of graft polymerizing polar monomer, oligomer or polymer byusing the ionizing radiation so as to reform a surface of the film, or aprocess of plasma-treating a surface of the film in a vacuum or normalpressure by using proper carrier and reaction gas so as to reform thesurface may be included in order to increase surface energy for thepurposes of increase in the impregnation rate of the organicelectrolytes and enhancement of the bonding force between the coatinglayer and the polyolefin film.

An adhesive component for improving the bonding force with the coatinglayer may be coated on the polyolefin microporous film before coatingthe coating layer. A monomer, oligomer or polymer material may be usedas an adhesive, and any material and process for improving the bondingforce may be used with a range that the permeability described in theclaims is not deteriorated. Preferably, the polyolefin microporous filmhas a porosity of 30 to 60%, a thickness of 5 to 30 μm, and an averagepore size of 0.01 to 0.5 μm. If the porosity or the thickness is toolarge, or if the average pore size is too small, it is not possible tosecure a passage for ions, and thus a resistance in the battery may beincreased. In reverse case, it is difficult to expect securing ofstability against the electrical short. In order to secure thesufficient stability in the battery, preferably, the polyolefinmicroporous film has a gas permeability is 1.5×10⁻⁵ to 20.0×10⁻⁵ Darcy,a tensile strength of 500 to 3,000 kg/cm², a closing temperature of 120to 140° C. and a meltdown temperature of 140 to 200° C.

The method of forming the thermally stable porous layer at hightemperature on one surface or both surfaces of the polyolefinmicroporous film is classified into a pore forming method byphase-separation and a pore forming method by extraction. The poreforming method by phase-separation is divided into vapor inducedphase-separation, thermally induced phase-separation and nonsolventinduced phase-separation. The present invention uses the nonsolventinduced phase-separation method in which a solution containing anonsolvent and a solvent is coated on one surface or both surfaces ofthe polyolefin microporous film, and the phase-separation is carried outas the solvent is dried, and then the nonsolvent is dried or extractedso as to form a porous structure. The method includes the followingprocesses.

The method of forming the thermally stable porous layer at hightemperature on the polyolefin microporous film includes:

(1) preparing a mixed solution by mixing organic or inorganic particlesand a nonsolvent of heat-resistant polymer with a solution in which theheat-resistant polymer is dissolved in a solvent;

(2) coating the mixed solution on one surface or both surfaces of apolyolefin microporous film;

(3) forming the thermally stable porous layer by drying andphase-separating the coated polyolefin microporous film; and

(4) removing the solvent and nonsolvent remained in the thermally stableporous layer by drying or extracting.

The heat-resistant polymer of the layer has a melting temperature or aglass transition temperature of 170 to 500° C. and is not limitedspecially if it contains aromatic ring in main chain. Preferably, theheat-resistant polymer contains one or more of polyamide, polyimide,polyamideimide, polyethylene terephthalate, polycarbonate, polyarylate,polyetherimide, polyphenylene sulfone, polysulfone and so on. It ispreferable to use polycarbonate or polyarylate which is relativelystable in the organic electrolytes.

A composition for the layer contains organic or inorganic particleswhich are selected according to various purposes of increase in theimpregnation rate of the liquid electrolytes with respect to theseparator, increase in the physical strength of the coating layer,increase in the porosity of the coating layer, increase in the heatresistance of the separator, and prevention of the electrical short bysecuring a space between electrodes upon an abnormal condition of thebattery. The organic or inorganic particles are not limited particularlyif they are stable electrochemically. The organic particles includingone or more of polyvinyldene fluoride (PVdF), polytetrafluoroethylene(PTFE), polyurethane, polymethylpentene (PMP), polyethyleneterephthalate (PET), polycarbonate (PC), polyester, polyvinyl alcohol(PVA), polyacrylonitrile (PAN), polymethylene oxide (PMO), polymethylmethacrylate (PMMA), polyethylene oxide (PEO), cellulose and so on, andthe inorganic particles including one or more of an oxide, a hydroxide,a sulfide, a nitride, a carbide and the like of at least one of metallicor semiconductor elements such as Si, Al, Ca, Ti, B, Sn, Mg, Li, Co, Ni,Sr, Ce, Zr, Y, Pb, Zn, Ba, or alone or a mixture of the organic andinorganic particles may be used.

If necessary, some additives for particular purposes, such asantioxidant, UV stabilizer, and antistatic agent, may be added to thecomposition of the coating layer within a range that the characteristicsof the separator are not deteriorated remarkably.

In order to enhance the heat resistance and strength of the polyolefinmicroporous film and also enhance the stability in the organiceletrolytes, the process of mixing monomer or oligomer having anunsaturated bonding group, and polymerizing and chemically cross-linkingthe mixture by using heat energy or ionizing radiation, or cross-linkingthe polyolefin alone or together with an initiator by using the ionizingradiation and the like may be included.

Concentrations of the organic or inorganic particle and theheat-resistant polymer, which form the final layer, in the solution tobe coated on the polyolefin microporous film are not limited specially,if it is proper to show the characteristics of the above-mentionedmicroporous composite film. However, the concentration of theheat-resistant polymer is preferably 1 to 50 wt %, more preferably, 2 to20 wt %. If the concentration of the heat-resistant polymer is less than1 wt %, since it is difficult to form the coating layer having asufficient thickness and uniform pores, it is not possible to enhancethe thermal stability of the separator. If the concentration of theheat-resistant polymer is more than 50 wt %, since it is difficult toform the coating layer having a sufficient permeability, the performanceof the battery is deteriorated due to increase of the resistance.

Further, the concentration of the organic or inorganic particle ispreferably 1 to 50 wt %, more preferably, 2 to 20 wt %. If theconcentration of the organic or inorganic particle is less than 1 wt %,it is difficult to achieve its purposes of increase in the impregnationrate of the liquid electrolytes with respect to the separator, increasein the physical strength of the coating layer, increase in the porosityof the coating layer, increase in the heat resistance of the separator,and prevention of the electrical short by securing a space betweenelectrodes upon an abnormal condition of the battery. If theconcentration of the organic or inorganic particle is more than 50 wt %,the bonding force with the polyolefin microporous film is deteriorateddue to relative reduction of the concentration of the heat-resistantpolymer, and thus it is difficult to secure the heat resistance of theseparator.

The heat-resistant polymer of the coating layer is dissolved in theorganic solvent and then coated in the form of a solution. The organicsolvent is not limited specially, if it can dissolve the heat-resistantpolymer, and includes one or more of N,N-dimethylformamide (DMF),N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc),dimethylsulfoxide (DMSO), benzene, toluene, phenol, cresol, pyridine,chlorobenzene, dichlorobenzene, dioxane, dioxolane, acetone,methylethylketone (MEK), cyclohexanone, chloroform, tetrahydrofuran(THF), dichloroethane, dichloroethyl ene, trichloroethane,thrichloroethylene, dichloromethane; (MC), ethyl acetate and the like.In order to efficiently carry out the phase separation and increase thedrying efficiency, it is preferable to use a solvent having a relativelyhigh vapor pressure and thus having high volatility.

Moreover, the organic or inorganic particles are dispersed in theorganic solvent. A higher polar solvent may be used in order to increasethe dispersive ability of the particles. For example, the higher polarsolvent may include water, alcohol, diol, ether, glycol, carbonate,ketone, phthalate and so on, and a mixture thereof.

The nonsolvent is not limited specially, if it can solidify theheat-resistant polymer of the coating layer and induce the phaseseparation. The nonsolvent includes water, alcohol, diol, hydrocarbon,ether, glycol, carbonate, ketone, phthalate, and a mixture thereof.

A content of the nonsolvent in the solution not limited specially, if itis proper to show the characteristics of the above-mentioned microporouscomposite film. However, the content of the nonsolvent is preferably 1to 50 wt %, more preferably, 2 to 30 wt %. If the content of thenonsolvent is less than 1 wt %, it is not possible to form the coatinglayer having the sufficient permeability, and thus the performance ofthe battery is deteriorated due to increase of the resistance. If thecontent of the nonsolvent is more than 50 wt %, the pore size may be toolarge, or it is difficult to obtain the uniform pores, and thus it mayexert an influence on the thermal stability of the separator. Further,it is preferable that the nonsolvent is selected from non-volatileliquids comparing with the solvent. This is caused by that the phaseseparation is carried out, as the concentration of the nonsolvent at thecoated solution layer is increased by the drying.

A method of coating one surface or both surfaces or an internal portionof the polyolefin microporous film with the coating polymer solutionprepared by above-mentioned method, which is not limited particularly,includes a bar coating method, a rod coating method, a die coatingmethod, a comma coating method, a micro gravure/gravure method, a dipcoating method, a spray method, a spin method and a mixed methodthereof. After that, a process of removing a part of the coating layerusing a doctor blade or an air knife.

During or after forming the coating layer on the polyolefin microporousfilm, a process of graft polymerizing polar monomer, oligomer or polymerby using the ionizing radiation so as to reform a surface of the film,or a process of plasma-treating a surface of the film in a vacuum ornormal pressure by using proper carrier and reaction gas so as to reformthe surface may be included in order to increase surface energy for thepurposes of increase in the impregnation rate of the organicelectrolytes used when applied to the battery.

Advantageous Effects

As described above, since the microporous polyolefin composite film ofthe present invention contains the organic or inorganic particles, ithas excellent permeability and high thermal stability at hightemperature, and particularly, since it has excellent stability of thecoating layer in high temperature organic electrolytes, it has a highmeltdown temperature and a lower shrinkage at high temperature.

Further, the microporous polyolefin composite film of the presentinvention has excellent quality uniformity and wide application range,and thus it can show an excellent effect when applied to a highcapacity/high power battery.

DESCRIPTION OF DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the following description ofpreferred embodiments given in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a photograph of a scanning electron microscope (10,000magnifications) showing a surface of a microporous film according to asecond example of the present invention.

FIG. 2 is a photograph of a scanning electron microscope (10,000magnifications) showing a surface of a microporous film according to afirst comparative example.

FIG. 3 a photograph of a scanning electron microscope (10,000magnifications) showing a surface of a microporous film according to athird comparative example.

BEST MODE

Hereinafter, the examples of the present invention will be described indetail with reference to accompanying drawings.

EXAMPLES

Characteristics of a microporous polyolefin composite film of thepresent invention are estimated by the following test method.

(1) Thickness of a Film and Coating Layer

A contact type thickness measuring, device having a precision of 0.1 μmwith respect to a thickness is used, and values that three points ormore in a TD and ten points or more in a MD are measured with respect tothe microporous polyolefin composite film are used. A thickness of thecoating layer is measured from a difference between a thickness of themicroporous film before the coating and a thickness of the microporousfilm after the coating. In case of the microporous film of which bothsurfaces are coated with the coating layer, a half of the differencebetween the thickness before coating and thickness after the coating isused as the thickness of the microporous film.

(2) Porosity (%)

A porosity is calculated by the following equation 1 using a rectangularsample of Acm×Bcm. In all of the samples, A, B is within a range of 5 to20 cm.

Porosity={(A×B×T)−(M÷ρ)÷(A×B×T)}]×100  [Equation 1]

wherein T=thickness (cm) of separator,

M=weight (g) of sample, and

ρ=a density (g/cm³) of resin.

(3) Pore Size and Particle Size

A pore size is measured using a porometer (PMI company) in a half-drymethod based on ASTM F316-03. An organic/inorganic particle size ismeasured from an apparent pore size calculated from a photograph of ascanning electron microscope with respect to the film surface.

(4) Gas Permeability (Darcy)

A gas permeability is measured using a porometer (CFP-1500-AEL of PMIcompany). In general, the gas permeability is represented by a Gurleynumber, but since an influence by the film thickness is not compensatedin the Gurley number, it is difficult to know a relative permeabilityaccording to a pore structure of the film. To solve the problem, thepresent invention uses Darcy's permeability constant. The Darcy'spermeability constant is calculated from an equation 2, and nitrogen isused.

C=(8FTV)/(πD ²(P ²−1))  [Equation 2]

wherein C=Darcy's permeability constant,

F=flow rate

T=thickness of sample

V=viscosity (0.185 for N₂) of gas

D=diameter of sample

P=pressure

The present invention uses an average value of the Darcy's permeabilityconstants in a range of 100 to 200 psi.

(5) Impregnation Rate of Electrolyte

After a test sample is kept at a room temperature and a relativehumidity of 50%, the sample is cut into a size of 10×10 cm, and aninitial weight (A) of the sample is measured. After the sample isimmersed for 1 hour in the electrolyte, the microporous polyolefincomposite film is taken out, and the electrolyte on the surface thereofis sufficiently removed with tissue paper, and then an impregnation rateis measured by measuring a weight (B) thereof. An average value withrespect to at least five samples is used as the impregnation rate, andthe impregnation rate is calculated by the following equation 3:

% impregnation rate=((B−A/A)×100  [Equation 3]

In the estimation of the impregnation rate, the electrolyte in which 1Mlithium hexafluorophosphate (LiPF₆) is dissolved in a solution thatethylene carbonate and dimethyl carbonate are mixed in a weight ratio of1:1 is used.

(6) Puncture Strength (N/μm)

A puncture strength is measured using UTM (Universal Test Machine) 3345fabricated by INSTRON company, when pressing the sample at a speed of120 mm/min. At this time, a pin has a diameter of 1.0 mm, and a pin tiphas a radius of curvature of 0.5 mm.

Puncture strength(N/μm)=measuring load(N)÷thickness(μm) ofseparator  [Equation 4]

(7) A Tensile Strength is Measured in Accordance with ASTM D882.

(8) For a bonding force, a 180° exfoliation bonding strength is measuredon the basis of JIS K 6854-2. The bonding strength is measured using UTM(Universal Test Machine) 3345 fabricated by INSTRON company, whenpulling the sample having a width of 25 mm at a speed of 100 mm/min. Anaverage value of the bonding strength generated upon the exfoliation isused.

(9) A shrinkage is obtained by measuring MD and TD shrinkages inpercent, after the microporous polyolefin composite film is kept for 60minutes at a temperature of 150° C.

(10) Shutdown Temperature and Meltdown Temperature

Shutdown temperature and meltdown temperature of the microporouspolyolefin composite film is measured in a simple cell which can measureimpedance. In the simple cell, the microporous polyolefin composite filmis interposed between two graphite electrodes, and electrolytes areinjected. An electrical resistance is measured, while temperature isincreased from 25 to 200° C. at a rate of 5° C./min by using analternating current of 1 kHz. At this time, a temperature in which theelectrical resistance is rapidly increased to a few hundreds to a fewthousands Ω or more is selected as the closing temperature, and atemperature in which the electrical resistance is again reduced to 100Ωor less is selected as the meltdown temperature. And the electrolyte inwhich 1M lithium hexafluorophosphate (LiPF₆) is dissolved in a solutionthat ethylene carbonate and propylene carbonate are mixed in a weightratio of 1:1 is used.

(11) Hot Box Test

A battery is fabricated by using the microporous polyolefin compositefilm as the separator. After an anode in which LiCoO₂ is used as anactive material and a cathode in which graphite carbon is used as anactive material are wound together with the separator and then put intoan aluminum pack, the electrolyte in which 1M lithiumhexafluorophosphate (LiPF₆) is dissolved in a solution that ethylenecarbonate and dimethyl carbonate are mixed in a weight ratio of 1:1 isinjected therein, and then the aluminum pack is sealed, whereby abattery is assembled.

The assembled battery is put into an oven, and the temperature isincreased to a temperature of 150° C. at a rate of 5° C./min, and thenwhile the battery is left for 30 minutes, a change in the battery isobserved and measured.

Example 1

In order to prepare the polyolefin microporous film, high densitypolyethylene having a weight average molecular weight of 3.8×10⁵ isused, and a mixture in which dibutyl phthalate and paraffin oil(kinematic viscosity at 40° C.: 160 cSt) is mixed at a rate of 1:2 isused as a diluent, and each content of the polyethylene and the diluentis 30 wt % and 70 wt %, respectively. This composition is extruded at atemperature of 240° C. using a dual-axial compounder having a T-die, andpassed through an area, of which temperature is set to 180° C., so as toinduce a phase separation, and then a sheet is prepared using a castingroll. The sheet is prepared by a successive bi-axial stretching methodin which a stretching rate is six times in each of a MD and a TD, and astretching temperature is 121° C. Herein, a heat-setting temperature is128° C., and a heat-setting width is 1-1,2-1.1. A final film has athickness of 16 μm and a gas permeability of 3.5×10⁻⁵ Darcy. A solutionfor forming the polymer coating layer is prepared by dissolvingpolycarbonate having a melting temperature of 231° C. in 1,4-dioxanesolvent and then adding nonsolvent of ethylene glycol monobutyl ether(EGMBE) and silica (SiO₂, average particle size of 400 nm) which issurface-treated with 3-methacryloxypropyltrimethoxysilane (γ-MPS). In acomposition of the solution, resin/particle/solvent/nonsolvent is4/10/78/8 wt %. One surface is coated by the bar coating method, andthen the film is dried in an oven of 60° C. for 30 minutes.

Example 2

The same the polyolefin microporous film as in the example 1 is used,and the solution for forming the polymer coating layer is prepared bydissolving polyarylate (PAR) having a glass transition temperature of201° C. in THF solvent and also adding alumina (Al₂O₃, average particlesize of 400 nm) and nonsolvent of pentanol. In the composition of thesolution, resin/particle/solvent/nonsolvent is 4/8/82/6 wt %. Onesurface is coated by the bar coating method, and then the coated film isdried in an oven of 60° C. for 30 minutes.

A photograph of a scanning electron microscope showing a surface of themanufacture microporous polyolefin composite film is illustrated in FIG.1.

Example 3

The same the polyolefin microporous film as in the example 1 is used,and the solution for forming the polymer coating layer is prepared bydissolving polycarbonate (PC) having a melting temperature of 231° C. inTHF solvent and also adding nonsolvent of pentanol and silica (SiO₂,average particle size of 400 nm) which is surface-treated with3-methacryloxypropyltrimethoxysilane (γ-MPS). In the composition of thesolution, resin/particle/solvent/nonsolvent is 4/8/82/6 wt %. Onesurface is coated by the bar coating method, and then the coated film isdried in an oven of 60° C. for 30 minutes.

Example 4

The same the polyolefin microporous film as in the example 1 is used,and the solution for forming the polymer coating layer is prepared bydissolving and dispersing polysulfone (PSf) having a glass transitiontemperature of 189° C. and silica (SiO₂, average particle size of 400nm) which is surface-treated with 3-methacryloxypropyltrimethoxysilane(γ-MPS) in THF solvent and then adding nonsolvent of n-butanol. In thecomposition of the solution, resin/particle/solvent/nonsolvent is4/8/72/16 wt %. One surface is coated by the bar coating method, andthen the coated film is dried in an oven of 60° C. for 30 minutes.

Example 5

The same the polyolefin microporous film as in the example 1 is used.Before forming the coating layer, plasma is discharged at atmosphericpressure for three seconds on the surface, on which the coating layer isformed, using nitrogen carrier gas and oxygen reaction gas. The solutionfor forming the polymer coating layer is prepared by dissolvingpolycarbonate (PC) having a melting temperature of 231° C. in THFsolvent and also adding nonsolvent of pentanol and silica (SiO₂, averageparticle size of 400 nm) which is surface-treated with3-methacryloxypropyltrimethoxysilane (γ-MPS). In the composition of thesolution, resin/particle/solvent/nonsolvent is 4/8/78/10 wt %. Onesurface is coated by the bar coating method, and then the coated film isdried in an oven of 60° C. for 30 minutes.

Comparative Example 1

In order to prepare the polyolefin microporous film, high densitypolyethylene having a weight average molecular weight of 3.8×10⁵ isused, and a mixture in which dibutyl phthalate and paraffin oil(kinematic viscosity at 40° C.: 160 cSt) is mixed at a rate of 1:2 isused as a diluent, and each content of the polyethylene and the diluentis 30 wt % and 70 wt %, respectively. This composition is extruded at atemperature of 240° C. using a dual-axial compounder having a T-die, andpassed through an area, of which temperature is set to 180° C., so as toinduce a phase separation, and then a sheet is prepared using a castingroll. The sheet is prepared by a successive bi-axial stretching methodin which a stretching rate is six times in each of a machine direction(MD) and a transverse direction (TD), and a stretching temperature is121° C. Herein, a heat-setting temperature is 128° C., and aheat-setting width is 1-1.2-1.1. A final film has a thickness of 16 μmand a gas permeability of 3.5×10⁻⁵ Darcy, and the polymer coating layeris not coated.

A photograph of a scanning electron microscope showing a surface of themanufacture polyolefin microporous film is illustrated in FIG. 2.

Comparative Example 2

The same the polyolefin microporous film as in the comparative example 1is used, and the solution for forming the polymer coating layer isprepared by dissolving polycarbonate (PC) having a melting temperatureof 231° C. in THF solvent and also adding silica (SiO2, average particlesize of 400 nm) which is surface-treated with3-methacryloxypropyltrimethoxysilane (γ-MPS), and the nonsolvent is notadded. In the composition of the solution, resin/particle/solvent isApr. 10, 1986 wt %. One surface is coated by the bar coating method, andthen the coated film is dried in an oven of 60° C. for 30 minutes.

[Comparative example 3]

The same the polyolefin microporous film as in the example 1 is used,and the solution for forming the polymer coating layer is prepared bydissolving polycarbonate (PC) having a melting temperature of 231° C. inTHF solvent and also adding nonsolvent of pentanol, and the particlesare not added. In the composition of the solution,resin/solvent/nonsolvent is 4/90/6 wt %. One surface is coated by thebar coating method, and then the coated film is dried in an oven of 60°C. for 30 minutes.

A photograph of a scanning electron microscope showing a surface of themanufacture microporous polyolefin composite film is illustrated in FIG.3.

Comparative Example 4

The same the polyolefin microporous film as in the comparative example 1is used, and the solution for forming the polymer coating layer isprepared by dissolving non-aromatic cellulose acetate having a glasstransition temperature of 190° C. in acetone solvent and then addingalumina (Al₂O₃, average particle size of 400 nm) and nonsolvent ofpentanol. In the composition of the solution,resin/particle/solvent/nonsolvent is 4/8/82/6 wt %. One surface iscoated by the bar coating method, and then the coated film is dried inan oven of 60° C. for 30 minutes.

Comparative Example 5

The same the polyolefin microporous film as in the example 1 is used,and the solution for forming the polymer coating layer is prepared bydissolving polycarbonate (PC) having a melting temperature of 231° C. inTHF solvent and also adding nonsolvent of n-butanol and silica (SiO₂,average particle size of 400 nm) which is surface-treated with3-methacryloxypropyltrimethoxysilane (γ-MPS). In the composition of thesolution, resin/particle/solvent/nonsolvent is 3.5/10/76.5/10 wt %. Onesurface is coated by the bar coating method, and then the coated film isdried in an oven of 70° C. for 20 minutes. Testing conditions of theexamples and the comparative examples and the results obtained therefromare represented in table 1 and table 2.

TABLE 1 unit Example 1 Example 2 Example 3 Example 4 Example 5Polyolefin microporous film — PE PE PE PE PE Heat Resin (%) PC (4) + PAR(4) + PC (4) + PSf (4) + PC (3.5) + resistant (concentration) SiO₂ (10)Al₂O₃ (8) SiO₂ (8) SiO₂ (8) SiO₂ (10) treatment Solvent (%) 1,4- THF(82) THF (82) THF (72) THF (76.5) (concentration) dioxane (78)Nonsolvent (%) EGMBE Pentanol Pentanol n-butanol Pentanol(concentration) (8) (6) (6) (16) (10) Manufacturing — Phase Phase PhasePhase Plasma + separation separation separation separation phase (adding(adding (adding (adding separation nonsolvent) nonsolvent) nonsolvent)nonsolvent) (adding nonsolvent) Thickness of Polyolefin μm 16.1 16.016.0 16.0 16.1 microporous film Thickness of coating layer μm 4.5 5.55.0 5.2 5.2 Bonding strength Kgf/cm 0.22 0.28 0.25 0.23 0.31 Puncturestrength N/μm 0.21 0.19 0.20 0.20 0.20 Tensile MD Kgf/cm² 992 932 978953 972 strength TD Kgf/cm² 793 728 770 742 768 Gas permeability 10⁻⁵1.9 2.2 2.1 2.0 2.2 Darcy Organic/inorganic particle μm 0.4 0.4 0.08 0.40.4 size Impregnation rate of % 152 143 161 147 153 electrolyteShrinkage, MD % 23 17 22 31 14 150° C., TD % 19 14 18 24 12 60 minutesShutdown temperature ° C. 136 135 135 136 136 Meltdown temperature ° C.175 >190 183 >190 >190 Hot box (150° C., 30 minutes) — Pass Pass PassPass Pass

TABLE 2 Comparative Comparative Comparative Comparative Comparative unitexample 1 example 2 example 3 example 4 example 5 Polyolefin microporous— PE PE PE PE PE film Heat Resin (%) — PC (4) + PC (4) Cellulose PC(3.5) + resistant (concentration) SiO₂ (10) acetate (4) + SiO₂ (10)treatment Al₂O₃ (8) Solvent (%) — THF (86) THF (90) Acetone (82) THF(76.5) (concentration) Nonsolvent (%) — — Pentanol Pentanol n-butanol(concentration) (6) (6) (10) Manufacturing — — Drying Phase Phase Phasewithout separation separation separation nonsolvent (adding (adding(adding nonsolvent) nonsolvent) nonsolvent) Thickness of Polyolefin μm16.0 15.9 16.0 15.9 16.0 microporous film Thickness of coating layer μm— 4.8 5.5 4.3 5.2 Bonding strength Kgf/cm — 0.30 0.29 0.23 0.08 Puncturestrength N/μm 0.25 0.21 0.18 021 0.19 Tensile MD Kgf/cm² 1308 1027 9211010 934 strength TD Kgf/cm² 1056 898 783 889 789 Gas permeability 10⁻⁵3.5 0.3 1.3 2.3 2.9 Darcy Organic/inorganic particle μm — 0.4 — 0.4 0.4size Impregnation rate of % 110 120 123 127 131 electrolyte Shrinkage,MD % 70 16 18 34 52 150° C., TD % 61 13 14 26 44 60 minutes Shutdowntemperature ° C. 135 135 136 135 135 Meltdown temperature ° C. 145 >190189 157 159 Hot box (150° C., 30 minutes) — Fail Pass Pass Fail Fail

Those skilled in the art will appreciate that the conceptions andspecific embodiments disclosed in the foregoing description may bereadily utilized as a basis for modifying or designing other embodimentsfor carrying out the same purposes of the present invention. Thoseskilled in the art will also appreciate that such equivalent embodimentsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

INDUSTRIAL APPLICABILITY

As described above, since the microporous polyolefin composite film ofthe present invention contains the organic or inorganic particles, ithas excellent permeability and high thermal stability at hightemperature, and particularly, since it has excellent stability of thecoating layer in high temperature organic electrolyte, it has a highmeltdown temperature and a lower shrinkage at high temperature.

Further, the microporous polyolefin composite film of the presentinvention has excellent quality uniformity and wide application range,and thus it can show an excellent effect when applied to a highcapacity/high power battery.

1-6. (canceled)
 7. A method of manufacturing a microporous polyolefincomposite film with a thermally stable porous layer at high temperature,comprising: preparing a mixed solution by mixing organic or inorganicparticles and a nonsolvent of heat-resistant polymer with a solution inwhich the heat-resistant polymer is dissolved in a solvent; coating themixed solution on one surface or both surfaces of a polyolefinmicroporous film; forming the thermally stable porous layer by dryingand phase-separating the coated polyolefin microporous film; andremoving the solvent and nonsolvent remained in the thermally stableporous layer by drying or extracting.
 8. The method of manufacturing amicroporous polyolefin composite film according to claim 7, wherein thenonsovent is selected from a group of water, alcohol, diol, hydrocarbon,ether, glycol, carbonate, ketone, phthalate and a mixture thereof. 9-10.(canceled)